1. Field of the Invention
The present invention relates to a dry etching method applicable to the production of semiconductor devices, and more particularly to a method of selectively dry-etching silicon nitride layers and silicon oxide layers.
2. Description of the Related Art
A typical silicon semiconductor device has its layers insulated by a film of silicon compound, especially silicon oxide (SiO.sub.x, where x is typically 2). The SiO.sub.x interlayer insulating film undergoes dry etching, which is a fully-developed technology that has been in use for mass production since the advent of 64 k DRAM.
Dry etching resorts to an etching gas composed mainly of fluorocarbon compound such as CHF.sub.3, CF.sub.4 /H.sub.2 mixture, CF.sub.4 /O.sub.2 mixture, and C.sub.2 F.sub.6 /CHF.sub.3 mixture, which offers the following advantages.
(a) The fluorocarbon compound contains carbon atoms which form on the surface of the SiO.sub.x layer, the C-O bond having a high interatomic bond energy, thereby breaking or weakening the Si-O bond. PA1 (b) The fluorocarbon compound forms CF.sub.x * radicals (where x is typically 3), which are the major etchant for the SiO.sub.2 layer. PA1 (c) The fluorocarbon compound provides good selectivity for the resist mask and the underlying layer, with a minimum deposition of a carbon polymer, if the C/F ratio in the etching reaction system is properly controlled. (The underlying layer denotes silicon layers such as silicon substrate, polysilicon layer, and polyside film.) PA1 553 kcal/mol for the Si--F bond PA1 465 kcal/mol for the Si--O bond PA1 440 kcal/mol for the Si--N bond
The silicon semiconductor device also has its layers insulated by a film of silicon nitride (Si.sub.x N.sub.y, where especially x=3 and y=4). The Si.sub.x N.sub.y layer also undergoes dry etching using an etchant which has basically the same composition as that used for the SiO.sub.x layer. While the etching of SiO.sub.x layers resorts to the ion-assisted reaction, the etching of Si.sub.x N.sub.y layers resorts to the radical reaction in which F* radicals play an important role. In addition, the latter is faster than the former. This can be expected based on the varying interatomic bond energies given below.
(taken from "Handbook of Chemistry and Physics", 69th ed. (1988), edited by R. C. Weast, published by CRC Press, Florida, U.S.)
The production of silicon semiconductor devices involves several steps for the highly-selective etching of SiO.sub.x layers and Si.sub.x N.sub.y layers. For example, the Si.sub.x N.sub.y layer on the SiO.sub.x layer undergoes etching for the patterning to define the element separating regions by the LOCOS (local oxidation of silicon) method. This etching needs to have an especially high selectivity under the condition that the pad oxide film (SiO.sub.2 layer) is made to be thin to minimize the bird's beak length.
On the other hand, as a result of recent devices becoming smaller and more complex than before, there has occurred an instance where it is necessary to carry out selective etching for the SiO.sub.x layer on the Si.sub.x N.sub.y layer as an etching stop layer to prevent etching damage. For example, recent devices have a thin Si.sub.x N.sub.y layer formed on the substrate surface to relieve the substrate from etching damage in the case of over-etching. They also have a gate insulating film of ONO structure (SiO.sub.x layer / Si.sub.x N.sub.y layer / SiO.sub.x layer), or they have an Si.sub.x N.sub.y layer laminated on the surface of the gate electrode. In these cases, it is necessary that the etching of the SiO.sub.x layer stop with certainty when it reaches the surface of the Si.sub.x N.sub.y layer.
For the highly-selective etching to be applied to layers formed on top of the other, it is desirable that they differ in the interatomic bond energy to some extent. Unfortunately, the SiO.sub.x layer and Si.sub.x N.sub.y layer have an Si--O bond and Si--N bond, respectively, whose interatomic bond energies are close to each other. Therefore, it is basically difficult to perform highly-selective etching on them.
Attempts have been made to establish a technique for such selective etching. There are some reports on the method of etching an Si.sub.x N.sub.y layer on an SiO.sub.x layer. In fact, the present inventors have disclosed in Japanese Patent Laid-open No. 142744/1986 a technique which employs as an etching gas a mixture composed of a fluorocarbon gas (such as CH.sub.2 F.sub.2 having a low C/F atomic ratio) and 30-70 mol% of CO.sub.2. A fluorocarbon gas of low C/F ratio forms CF.sub.x..sup.+ (especially x=3) as an etchant for the SiO.sub.x only through recombination of F*. If this system is supplied with a large amount of CO* which captures F* and removes it in the form of COF, the formation of CF.sub.x.sup.+ decreases and hence the etching rate for the SiO.sub.2 layer decreases. On the other hand, since the Si.sub.x N.sub.y layer undergoes etching by F*, the etching rate for the Si.sub.x N.sub.y layer remains almost unchanged even though the amount of CF.sub.x.sup. + decreases due to the addition of copious amounts of CO.sub.2. The consequence is the selectivity for the two layers.
Another etching technique is reported in "Proceedings of Symposium on Dry Process", vol. 88, No. 7, pp. 86-94 (1987). This technique is characterized by feeding a chemical dry-etching apparatus with NF.sub.3 and Cl.sub.2 and carrying out etching for the Si.sub.x N.sub.y layer on the SiO.sub.x layer by utilizing FCl which is formed in the gas phase by microwave discharge. The fact that 55% of the Si--O bond energy is ionic whereas 30% of the Si--N bond energy is ionic suggests that the chemical bond in the Si.sub.x N.sub.y layer is similar to the chemical bond (covalent bond) in single-crystal silicon. Therefore, the Si.sub.x N.sub.y layer is subject to etching by F* and Cl* radicals dissociated from FCl, whereas the SiO.sub.x layer is immune to etching by these radicals. This is the reason for the high selectivity.
As mentioned above, there are reports on several techniques of selective etching of the Si.sub.x N.sub.y layer on the SiO.sub.x layer. These techniques are a natural consequence of the fact that etching of the Si.sub.x N.sub.y layer by radical reaction is necessarily decelerated as it reaches the SiO.sub.x layer. A disadvantage of these conventional techniques is that the process employing FCl (or the radical reaction) involves inherent difficulties with anisotropic etching.
By contrast, only a few techniques have been disclosed on selective etching of the SiO.sub.x layer on the Si.sub.x N.sub.y layer because it is more difficult to establish a desired selectivity than in the case where the two layers are reversed. The reason for this is that etching of the SiO.sub.x layer by an ion-assisted reaction inevitably forms radicals in the reaction system and these radicals accelerate the etching rate when the underlying layer (or the Si.sub.x N.sub.y layer) is exposed.
Recent technical advancement has, however, achieved this object, namely by use of a new plasma source which generates a high-density plasma with a smaller amount of radicals. An example is reported in Proceedings of the 43rd Symposium on Semiconductors and Integrated Circuits Technology, p. 54 (1992). According to this report, etching is carried out by means of induction coupled plasma (ICP) of C.sub.2 F.sub.6 (hexafluoroethane) gas for the SiO.sub.x layer (formed by the TEOS-CVD process) on the Si.sub.3 N.sub.4 layer (formed by LP-CVD process), so as to make a connecting hole which partly overlaps with the gate electrode. Etching in the reported technique occurs presumably due to CF.sup.+ formed from C.sub.2 F.sub.6 by dissociation in the high-density plasma. The high selectivity stems from the fact that etching deposits a fluorocarbon polymer of a low C/F ratio and the carbon atoms in the polymer combine more readily with the oxygen atoms in SiO.sub.x than with the nitrogen atoms in Si.sub.x N.sub.y, with the result that they are removed from the surface of the SiO.sub.x layer but they accumulate on the surface of the Si.sub.x N.sub.y layer.
The foregoing technique seems to be promising but has the disadvantage that it lacks a stable selectivity. For example, it is reported that the selectivity is infinite for the flat part but is 20 or above for the corner part. Presumably, the fluctuation in the selectivity is due to F* radicals, resulting from an extremely dissociated C.sub.2 F.sub.6 .